[FIX] mrp: quantity done on stock.move set as 0 when producing

To reproduce:
    1. create a BOM with analytic account
    2. create a MO to produce 10 of the above product
    3. change producing qty to 5, SAVE the form.
    4. change producing qty to 10, SAVE the form.

It happens because the server receive at the same time a new
`stock.move.line` creation and a quantity_done to write on the
`stock.move`.

closes odoo/odoo#86431

Signed-off-by: Arnold Moyaux <arm@odoo.com>